The present invention relates to ultrasound imaging, and more particularly, to a method and apparatus for enhancing ultrasound images by reducing speckle.
Ultrasound imaging is an attractive modality for numerous diagnostic procedures because of its non-invasive nature, relatively low cost, and lack of radiation exposure. Medical ultrasound images are typically produced by generating an ultrasonic sound wave traveling in a known direction, known as a scan line or a vector, and observing the echoes created when the sound wave is scattered or bounces off of the boundaries between regions of differing density in the body. For any given direction of the ultrasound beam, the image pixels are generated by plotting a dot whose brightness is proportional to the echoes"" amplitude at a coordinate whose location is a function of the time after a short ultrasound pulse is sent in the direction of the scan line being measured.
When forming images with coherent radiation, the desired distribution of image energy is subject to undesirable random modulation. This random distribution of energy is known as xe2x80x9cspecklexe2x80x9d and is manifested in visual images as flecks of random intensity and size distributed across the image. Speckle arises from constructive and destructive interference due to random phase cancellations and additions of the coherent field which is scattered by the coherently illuminated object. The power spectrum of speckle depends upon the spectrum of the coherent signal carrier, the texture or spatial distribution of scatterers in the field, the size of the irradiated object volume, and the transfer function of the receiving and imaging system.
The speckle caused by random tissue scattering and ultrasonic images may overshadow delicate tissue structures and degrade the image contrast. Frequency compounding is a well-known way to reduce speckles and hence enhance contrast resolution. In frequency compounding, images with different frequency characteristics are summed incoherently. In the experience of these inventors, existing frequency compounding methods for speckle reduction suffer from resolution degradation due to summation of detected narrow-band signals. These narrow-band signals are typically obtained by narrow-band filtering on receipt. What is needed is a high-resolution compounding method and apparatus.
Broadly, it is the object of the present invention to provide an improved method and apparatus for ultrasound imaging.
It is another object of the present invention to provide an improved ultrasonic imaging method and apparatus that can distinguish tissues while reducing the amount of speckle that overshadows delicate structures and degrades the image contrast. It is yet another object of the present invention to provide such a method and apparatus while using wide-band frequency compounding with tissue-generated harmonics. These and other objects of the present invention will become apparent to those skilled in the art from the follow detailed description of the invention and the accompanying drawings.
The present invention is a method and apparatus for smoothing the speckle pattern and increasing contrast resolution in ultrasound images. Compared to other frequency compounding techniques, wide-band harmonic frequency compounding reduces speckle noise without sacrificing the resolution. Compared to spatial compounding, wide-band harmonic frequency compounding is more robust against tissue motion because sequential vectors rather than frames are summed together for compounding. The method and apparatus of the present invention is implemented by transmitting two or more firings, combining two or more of the firings coherently to extract the tissue-generated harmonic components, detecting the outputs of the coherent sums and detecting one or more firings before coherent sum, and finally combining all detected outputs to form the compounded image.
The method and apparatus of the present invention sums wide-band fundamental and wide-band harmonic images after detection to form a compounded image. Unlike other frequency compounding methods, both transmit and receive signals are wide-band and no narrow-band filters are necessary. Multiple firings with two or more different transmit waveforms are transmitted to each focal zone.